Question

The rate of reaction is enhanced in the presence of light. What does this imply?

Answer 1

Some of the chemical reaction rates are enhanced by light. For example photosynthesis, free radical reactions, photography

1. 6H₂O + 6CO₂ + Light ----------------> C₆H₁₂O₆  + 6O2

2. 2AgCl + Light ---------------> 2Ag + Cl2

3.CH₃Cl + Cl + Light -------------> CH₂Cl₂  + HCl

   4. H₂ + Cl₂ + Light -----------------> 2HCl

This type of reactions are known as photochemical reactions.

The energy of light is sufficient to break the bonds to initiate the chemical reaction. i.e. the light has enough energy to overcome activation energy.

From example 1, The chlorophyl pigment in plants absorbs the visible light and initiates the chme

From example 2, AgCl is sensitive to light, so it is used in photographic films.

When the light hits the AgCl film it reduces Ag⁺ ----> Ag and oxidise 2Cl^- -----> Cl₂

From examples 3&4, These are examples of free radical reactions, when the UV-Vis light hits the reactants, it forms free radicals by breaking the H-H, H-CH₂Cl bonds.Then CH₂Cl radicals combines with Chlorine radicals by forming CH₂Cl₂. The reactions goes on until all the hydrogens are replaced by chlorine. these are very fast reactions.

Hence, we can conclude that some of the chemical reactions are carried in light. all these reactions are independent of concentration of reactants. So, they are Zero order reactions.And the speed of the reaction increases with increase in intensity of Light used.